77 Concerning the meaning of this symbol, which was known even to the earliest Fathers and ecclesiastical writers, St. Thomas Aquinas, echoing something of their words, writes as follows: "From the side of Christ, there flowed water for cleansing, blood for redeeming. Hence blood is associated with the sacrament of the Eucharist, water with the sacrament of Baptism, which has its cleansing power by virtue of the blood of Christ."(81)
Source: Haurietis Aquas (Vatican.va)